The present invention is relevant to a technical field of measurement of weak magnetic fields and more particularly it relates to a so-called SQUID magnetometer using a SQUID (superconducting quantum interference device) and a system for biomagnetic measurements using the SQUID magnetometer.
The SQUID constructed of a superconducting circuit including Josephson junctions is used for detection of weak magnetic fields. The SQUID is typically applied to a SQUID magnetometer using a DC-SQUID and the SQUID magnetometer has in general a construction as shown in FIG. 10. Referring to FIG. 10, a SQUID ring 7 is coupled magnetically with an input coil 8 and a modulation coil 9. Generally, the components 7, 8 and 9 are ganged to constitute a SQUID. The SQUID ring 7 is supplied with a DC bias current from an electronic circuit 15 and the modulation coil 9 is applied with a modulation signal and a feedback signal.
An input signal indicative of a magnetic flux passing through a pickup coil 60 is applied to the input coil 8.
These components of the input coil, modulation coil and SQUID ring may be formed on a single chip through the use of thin film technology and a resulting structure can be called a planar type SQUID. U.S. Pat. No. 4,761,611 discloses a SQUID gradiometer using a planar type SQUID. In this literature, a gradiometer coil of, for example, linear differential type is used as a pickup coil and connected by a superconducting cable to the SQUID contained in a magnetic shield case. Most typically, the SQUID magnetometer is constructed as in this literature, wherein the SQUID and the pickup coil are separately provided and the two are interconnected through the superconducting cable. On the other hand, there is disclosed in Biomagnetism '87 (1988), pp. 446-449 an example of an integrated-type SQUID magnetometer wherein a SQUID and a pickup coil are both formed on a single substrate through the use of thin film technology. This integrated-type SQUID magnetometer is not provided with any magnetic shield and is used, along with an object to be measured, in a magnetic shield room.